Electron gun



J. A. HALL ELECTRON GUN Filed Aug. 14, 1955 Fig.2.

MIJ@ i Fig.3.

INVENTOIR James A.II0|II.

BY l

ATTORNEYy 9 I I l a I r'lld'l'lrll.'

BILII WITNESSESI United Safes Patent@ ELECTRGN GUN Application August14,1953, Serial'No., 374,240 5 Claims.` (Cl. 313-82) My inventionrelates to cathode-ray tubes and infpar ticular relates to animprovement in the` cathode-ray guns which project electron beams ontothe fluorescent screens of such tubes. n

The electron beams in cathode-ray tubes are'proj'ected by an assemblageot electrodes known as`an electron gun such as is diagrammed in Fig. lwhich comprises` a `heated cathode l having a control electrode 2enclosing its electron-emissive end-portion except forl an aperture4 3.Facing the control electrode 2 is ascreen-electrode 4 comprising a disc5 with an aperture 6 aligned with aperture 3,. and a cylindrical wallwhich may have a rolled Aedge 7. Facing the wall 7 is the circular edgeof another cylindrical member 9 which may be .joined tocylindricalmember 13 by flanges 11 and 12. The member 13 is -closed at `its remoteend by aV disc with an aperture`17 `located on the axis of member 13. Toprovide'for an .ion trap `of a type now widely used in televisionpicture itubes, the rim of the screen-grid electrodewally'? and:adjacent edge of member 9 may be slanted at an angle of abouti() to l3degrees relative to theV plane perpen- (dicular to the axis of thescreen electrode 4. Theelectrostatic field produced between theseelectrodes at'ythe normaloperating voltages bends the unwanted beamiofnegative ions away from the axis of electrode 9 sothat .it cannot'passthrough the aperture "17, andl'th'ere'fore Jeannot reach and'damage thefluorescent `screen of tthe picture tube. The electron beam is caused'tofollow .a curved path through the aperture 17 by thefaction of :amagnetic field inthe directionperpendicular tothe 4plane' of thediagram. This magnetic eld'iscustomarly supplied by a magnet whose polepieces 'are located approximately over the screen-grid electrode4.'-f`The path yof the heavier negative ions is little affected by this-magnetic` eld. i

Coaxial withV cylinder 13`but spaced therefrom'by a *gap is a cylinder14, closed at its nearer end-by a disc fhav'ing an aperture` 13 locatedAon itszaxis. Thefdiscs in `:parts 13 and 1d may have'various shapes,`but areusually -of the dished shape indicated in Figure 2; The cylindersyof -parts 13 and 14 may be parallel sided `or1 may have 'asection ofsmaller diameter near the :closed ends of .these parts.

Surrounding the gap between .cylinders 13 and11'4 is Va larger`cylindrical sleeve 16 called the focussingl electrode. Sleeve 16 isinsulatingly supported coaxiallyyand :approximatelysymmetricallyrelative to the gap; between *cylinders 13 and 1e. In somedesigns,` af-similarly lo- Ycatedring or discwith centrallylocated-aperture is lprovidedinstead of the sleeve 16 toproducethefocussing lteld.

`type tubes in the receiver. Thus the range of 'adjustment ice The outerend of'cylinder 14 customarily has a wide flange which'is provided withsprings 15 of flexible metal which engage the walls of thecathode-raytube and assist in` positioning 'the cathode-ray gunjstructure`centrally, therein. Wh'ilecylinders 9.and 13 may becoaxial, they are'customarily joinedso that their axes intersect at an anglefof 3 to`5.5degrees, crossing `in the plane of the flanges or other joining means,so that theelectron beam after completing its curved `path yin theregion of the magnetic and axially unsymmetrical electrostaticteolds,`wil1 follow aM path along theaxes (of parts 13; 14 and16. "Thecylinders 27 4, 9, 13 and V14 are usually all of the saine insidediameter, the cathode 1 is insulatingly supported within the controlelectrode 2 andall electrodes are providedwith leads by whichelectrical. potentials may be' impressed on them from voltage sources.outside the cathode-ray tube. The above-described assemblage ofcylinders constitutes an electron-lens systemby 'which the electronsemanating from' cathode 1 may be focussed into a spot or"smalldiameteron theluorescent screen 19 at the remote endof thecathode-ray tube f2() which kforms a vacuum-tightenclosurefor therabove-,described structure as widely used today in televisionpictureptubes. The electron `beam is projected along .the axes ofelectrodes 13,414 and 16 and is then deected `by two setsof mutuallyperpendicular magnetic dellecting coilsi21and 22 to scan the picturescreenrll in a mannervnow `well known.

Theelectron optical characteristics of this assemblage dependsupon therelative `dimensions andspacings "of the component cylinders, sleevesand diaphragm` apertures,` and onl the magnitudes of thelelectricalpotentials limpressed on them. lt may be noted that.` dimensions andmagnitudes` in this electron-optical system are relative, so that anassemblage inwhich all dimensions are the same as those of anotherassemblage .multiplied by the same factorwill produce vsimilaropticalpatterns in `the latter provided it is supplied withelectricalpotentials fwhich are the same as those` impressed ontheformer.

The dimensions in the Fig. l assemblage..may therefore *be expressed asratios to the inside diameter D. of cylinder 2. While this or a similarsystem is/widely used in ymost recent electrostatically focussed picturetubezguns,

a number of undesirable features `have been found `which l `willnowfpoint out.

A. The bias voltage on control electrode 2.` isdmade `manually variableto enable control of picture brightness by varying the current in theelectron beam., `The potential needed for cylinder 16 to maintain thebest4 spot focus, hereinaliter called they focus voltage, becomesrapidly more negative as the picture brightness, Vand `hence beamcurrent, is increased. This'tocussing voltage is "customarily derivedfrom the supply ywhich furnishes 250 to `350volts D. @for anode powerfor `the receiving available may be insulrcient to `maintain the bestfocus with variation or picture brightness.

' B. Thevoltage applied to control electrode 2 is modu- -lated by thesignal from the'video amplifier ofthe television receiver to vary thebrightness of the fluorescent spot on the picture `tube screen by`varying tliebeam current asi-each picture element is scanned; In tubes.using electrostatically focussed` guns of the prior: art-` the ,size ofthe uorescent spot becomes` rapidly: larger: .and

the detail in the reproduced picture less distinct as the electronbeamcurrent is increased. This effect, known colloquially as blooming,causes loss of detail in picture highlights. I television reception inweak signal areas, stray signals originating in the receiver circuitscause a random pattern of white dots to be superimposed on thereproduced picture. This type of interference is known to workers in thefield as snow. The increase of fluorescent spot size with increasingbrightness then becomes especially undersirable since the white dotswill be large and much more objectionable to the viewer.

C. The potentialapplied to electrodes 9, 13, and 14, hereinafter calledthe anode potential, which is usually between l kv. and 18 kv. positivewith respect to the picture tube cathode, is subject to wide variationduring receiver operation. This variation is due both to fluctuations inthe voltage supplied by the domestic powersupply lines and to variationsin the load conditions within the television receiver since the anodepotential supply has very poor regulation. A drop of 1000 or more voltsfor a 200 microampere change in picture tube anode current is typical ofpresent practice. The anode supply voltage may also vary during the lifeof the receiver due to the gradual deterioration with life of othertubes and components in the receiver.

These variations tend to degrade picture definition since the focusvoltage required to maintain best picture definition becomes morenegative as the anode potential is decreased, in the order of 50 voltsfor every kilovolt change in the anode Voltage.

This effect becomes much more objectionable in light of A and B above,since these effects all tend to make the focussing voltage required morenegative. Further, the causes listed tend to add, since lowering theanode voltage tends to make the picture dim, while attempts to increasepicture brightness by changing the bias voltage applied to the controlelectrode to increase the tube current will further decrease the anodevoltage because of the poor power supply regulation.

D. The focus voltage required becomes rapidly more negative as thevoltage applied to the screen grid electrode 4 is increased. A 100 voltchange in this voltage will cause a 200 volt change in the focusvoltage. The percentage of the electrons leaving cathode 1 which finallyarrive at the fluorescent screen is a function of the operatingvoltages. At high brightnesses, the electron beam diameter in the gunmay exceed the diameter of aperture 17, and further attempts to increasethe brightness will increase the percentage of electrons which areintercepted by the end of electrode 13. For a given current of electronsfrom the cathode, the beam diameter at this point is increased if thevoltage on screen-grid electrode 4 is increased, or if the anode voltageon electrodes 9 and 13 is decreased. Since high picture brightness isdesired and since only the electrons which reach the fluorescentscreen'contribute to picture brightness, the electron gun must bedesigned so that little if any of the electron beam is intercepted byelectrode 13 under the least favorable conditions of operation expectedin the field.

E. To find'a good market, cathode-ray tubes must fit a wide variety ofdifferent television receivers, and the magnitudes of the screen-grid(electrode 4) and anode (electrodes 9, 13, 14) supply voltages varieswidely between television sets of different manufacturers. Typicalranges for these voltages are 275 to 500 volts for the screen-gridvoltage and l0 to 18 kilovolts for the'anode voltage. Furthermore, thepicture brightness requirements considered necessary by various receivermanufacturers range from 12 to 70 foot lamberts. Because variations inpicture brightness, screen-grid voltage and anode voltage all change thefocus voltage required by the picture tube, and because the range offocus voltage supplied in a television receiver is small (or zero insome cases) it was impossible todesign one electrostatically focussedcathode-ray tube which would perform satisfactorily in all receiversdesigned for this type oftube. As a result, cathode-ray tubemanufacturers find it necessary to build two or three tubes actuallydiffering from each other for each nominal tube type.

F. The magnetic field impressed near the screen-grid 4 as part of theion-trapping device is of non-uniform intensity across the regiontraversed by the electron beam; in electron guns of prior design, thecross section of the beam is distorted from a true circle, the shape ofthe light spot produced on the picture screen is distorted from a circleand picture resolution is degraded. Both the uniformity and strength ofthis magnetic field vary between sets of different manufacture, so thata compensating non-uniform electric field cannot be used to correct thisbeam distortion.

G. In the electron guns described, slight non-uniformities in theelectric field between electrodes 4 and 9 distorted the cross section ofthe beam from a circle, with loss of picture definition. These fieldnon-uniformities were both those caused by slight distortions of electrodes 4 and 9, and the general field asymmetry in this@ region requiredfor the ion-trapping action.

One object of my invention is accordingly to provide l a new andimproved structure for cathode-ray tubes.

Another object is to provide a new and improved type? of electron gunfor cathode-ray tubes.

Another object is to provide a new and improved type* ofelectrostatically focussed electron gii for cathode# ray tubes.

Another object is to provide a new and improved type" of electron gun ofthe socalled low voltage? electro-A statically focussed type forcathode-ray tubes.

Another object is to provide a cathode-ray gun of this@ type in whichthe focussing of the beam shall less sensitive to variations of anodepotential relative tofl cathode than cathode-ray guns of the prior art.

Another object is to provide a cathode-ray gun of this type in whichvariation of the focus voltage required to compensate for variations inthe screen-grid voltage shall be greatly reduced.

Another object is to provide a cathode-ray gun of this type in whichvariation of the focus voltage with bias voltage of the controlelectrode shall be smaller than that of the prior art.

Another object is to produce a cathode-ray gun in which variation of thefocus voltage required to compensate the effect on beam focus ofvariations in picture brightness shall be less than in conventionalcathode-raf.I tubes operating in conventional receivers.

Another object is to provide a cathode-ray gun in which the size of thelight spot on the picture-screen is smaller for a given gun length thanin cathode-ray guns of conventional type.

. Another object is to produce an electrostatically focused cathode-raygun in which substantially all focussing occurs'- in a single electronlens.

Still another object is to produce a cathode-ray gun in which focussingin the electron lens comprising the screen' grid and the first anode isreduced virtually to zero.

Still another object is to produce a cathode-ray gun in which thepercentage of the electrons emitted by the 'cathode which arrive at thepicture screen is little affected by variations in the anode voltage orthe screen-grid voltage.

Yet another object is to provide a cathode-ray gun which is lesssensitive to variations in the magnetic field comprised in the ion trapthan are prior art cathode-ray guns.

Yet another object is to provide a cathode-ray gun in which largertolerances in the symmetry of the electron lens comprising thescreen-grid first anode may be permitted than in cathode-ray guns of theprior art.

Still another object is to produce a cathode-ray gun which is lesssensitive to non-uniformity within field of the iontrap magnet than arecathode-ray guns of the prior art.

, Yet another objectis toproducea cathod'efraygun `in which thefocussing effect at `the rst electronglens, adjafl cent1 the screen-gridelectrode, is reduced and the focus` sing effect of the second electronlens, between theanode cylinders and focussing electrode, is normal; j

Other objects of my invention will become apparent upon reading thefollowing description taken in connection with. the drawings in which:

Figure 1 isa schematic view showing the relation ofthe cathode-ray gunembodying the principles of my invention to the otherstructuralelements'A of a cathode-ray tube.

Fig. 2is a schematic view partly raygun `of .the prior art; and

Fig; 3 Yis asimilar view` of a cathode-ray gun embodyingthe principlesof my invention;

Fig; 1 is believed to have already been described sul-A ciently so thatfurther description of it isA superfluous.

Referring toFig". 2 the cathode 1"c`ornpr`ise`s a" metal cylinder 30having a cupped end 33fsurfaced `with thefr'r'n ionically-emissivematerial which is heated byian'electri'cal heater-winding 23. Thecathode is supported by a` ceramic collar from" the control electrode 2,which has its end'facing cuppedend 22 closed exceptfor`central'"aperture3.` The distance from the emissive surface on cathode'1"t`o the aperture 3 isabout 0.01 D; the other important dirnen-A sionsVof-cathode 1 and control electrode 2 are tabulatedA below.

Facing the aperture 3 lin control electrode 2 is `the centralraperture 6in the lat end of screen-grid 4,: and ,facing 30 the otherV end ofV thelatter is the open end ofjcylindiicalf rst-anode-section 9; controlelectrode 2, 'screen-gridf4 and rst-anode-section 9 are rigidlyintercnnectedby tivov or `more rods of insulating material 25, 26 towhich 'their'` support-arms 27 are attached. The adjacent edgesvbfelec-i trodes 4 and 9`are in parallel planes inclined to 'a plane`perpendicular to the axis of screen-grid 4 by an angleof about, 10 to 13degrees, and the edge of screen-grid '4' inlay"l bei rolled over asshown or may be meelycut and pol'- ished to remove sharp edges and burrswhich mightlcaus'ej local distortionsof the electrostatieeld. "y t" Theright-hand side of rst-anode-section 9 isY joined by flanges 11,12 orsorne other suitable joining means tfolfsec-` tion 13 which has itsremoteend closed except' for af central aperture 17.` which is alignedwithvthe" central ap j ture 1S in the closed end ofanode-.section 14."The corn mon" axis'of electrodes 4 and 9 may be inclined byabout;q 3to`5.5 degrees to the common axis of electrodes` 13,"147` and'16,intersecting the latter at a point in `the planeofthe ariges or otherjoining means 11 12. T he anode-sections 143"a`nd:14 Yare electricallyinterconnected, and are` lgidly v` interconnected mechanically by a setof insulatingy i 28, 29 to which their support arms 31 are attached.. Asleeve or focussing electrode 16atxed-tosupport rods 28,` 29 surroundsthespagebetweenapertures 1] and 18. A wide ange 32 to which are attachedflexible spring-plates isprovided at the right-hand. end ofanode-section lat.V The apertures 3, 6, 17 and 18.are positionedlongthe' curved path which` electrons` will. travel through theVl'elec'- trostatic fields of these electrodes and the magnetic' fieldof an ion-trap magnet (not shown) mounted outside the tube nearelectrode 4.

In-leads through the wall of tube 2,0 make it possible to impress thedesired potentials onthe electrodes 111,12, 4,'9, 13,14, 16`and 23. Thuscathode 1 may he grounded; control electrode 2 may be impressed withsignal voltages and with a variable negative bias to control brightnessof the picture on Screen 19; screen-grid electrode 4 in section of acathodewith a Xed position voltage usually between 250 and 500 volts;electrodes 9, 13 and 14 with the anode supply voltage of 10 to 18kilovolts; and sleeve 16 with a focus voitage usually variable over aportion of the range from l00 to +450 volts.

For guns of the prior art, the relative dimensions of theseelectrodesare expressedfas ratios to-thediameter D of lcylinderZ.

Cylinder 'Diameter Length i Voltage outsraayl.. 0.72. 16.

i Inside, 1t--- Y 1-30.

Inside, 1, `0.71 (maximum ng +275 to 500V.. `-Ins1de,i1 f-41.8rmarirnumningun. u +1 0to 181m` Cylinder Inside Length Voltagediameter 1` .71 1otsrsknovons.`

1 1,02 10 to llkilovolts. 1. 3 j 1 4 .-*100` A.rofl-450.

Aperture` Inside u l tf i diameter between j e" 'length" 2- 4 0.045 j 4-9. .2 i ra-ifr .75,

In a typical-case the distance between aperture 18 and the" center 'ofa" 21 incli"diame`t`er fluorescent picture; screen' is about 18IVoltages listed are potential abovey the cathode for' an assemblagein'whi'ch D is 0.50 inch.

The electron-'optical system `in`Fi`g. 2 produces focusing at two pointssubsequent` to' the cathode lens; the

the'focal lengthof the electron lens formed'in andbe' tween electrodes 4and 9 by reducing the curvature of the equipotential surfaces of 'the"electric "'eld within the; screen-grid 4 wherernost of rthe focussingeffect of this lens resides. Thisfchangeof dimensions` also reduces theobjectdistance for 'thislens so that it becomes small compared to thefocalV length and makes they focussing etect of the lens negligible.

The above-mentioned increases'of focal length of the lens' resl'tsffro'rreducingV theft curvature of "the equipot'e'nt'ial surfaces-on the lowpotential side' o f the lens.

VAny' 'other methods 'of-reducing"thisl curvature and/or increasingv thefocal'length are withinmthe broad scope of rny invention.' l l 'Theregion 'of the electron gun betweenjaperturesxl'l and 18 which isintended 'to yfocus the bearnlmust then be`s1ightly changed ina mannerknown to workersl in,"

the" fieldtoincr'ease' thestre'n'gtli of this lens.

'The following advantageous results are attained by thismodificatio'noftheelectrnguii; "l f 1; The change ofthelfocussingfvoltage required when picture brightn'ess changes isgre'atlyfreducedi i. e. the

chang of2'50 to 300 volts per' 100 microarnperes change of beam. currentin present cathde-r'a'ybes" is' dropped 2. The adjustment of focussingvoltage required in present tubes with changes in screen-grid voltage isreduced to approximately one-sixth,

3. The adjustment of focussing now required whenv i anode voltagechanges, amounting to about 5 percent ofY the latter, drops to around 3percent.

4. The defocussing of the electron beam consequent upon the inevitablemomentary variations in intensity from point to point in the picture isgreatly reduced.

5. The cathode-ray spot size at the picture screen is decreased comparedwith conventional cathode-ray guns of the same length thus increasingpicture resolution in the ratio of from 800 to 1000 lines. This enablesreceiver designers to attain desired resolution without the necessityfor providing supply voltages outside usual values.

6. The percentage of electron beam current which reaches the picturescreen will be little affected over a wide range, by the magnitude ofthe screen-grid and anode voltages.

7. Because the focussing action'of the lens between the screen-grid andthe first anode has been greatly reduced, ellipticity of cathode-rayspots on the screen is nearly eliminated and asymmetry in the electronlens formed by the screen-grid and first anode produces less aberrationand astigmatism in the cathode-ray spot on the picture screen.

8. Non-uniformity and astigmatism in the cathode-ray beam arising fromthe departures from uniformity in the magnetic field of the ion-trapmagnet are greatly reduced when the focussing effect of thescreen-grid-tofirst-anode lens is reduced by my construction.

9. As a consequence of these advantages the same tube may be marketedfor use in makes of receiver having anode voltages from 10 to 18 kv. andscreen-grid voltages from 250 to 500 volts, Vwhich of our presentknowledge includes all the receiver designs which are now sold or havebeen sold for useY with low voltage electrostatically focussedcathode-ray tubes.

, Fig. 3 shows an electron gun in accordance with my invention with thelength of the screen-grid reduced as just described. The remainingdimensions of the gun structure may be those specified in describingFig. 2 and guns operating satisfactorily in substantially all picturereceivers employing low voltage electrostatically focussed,

cathode-ray tubes will. result. However, it may be found desirable todepart from the dimensions specified in the foregoing tabulation, and Iwill therefore state in exemplification that the gap between screen-grid4 and anode 9 may vary between 0.20 D and 0.24 D; the angle of the planeof the inclined rim of screen electrode 4 relative to a plane normal tothe axis of screen-grid 4 may lie Vbetween ten and thirteen degrees;that the angle between the axes of cylinder-sections 9 and 13 may varyfrom 0 to 5.5 degrees; that the rolled rim on screen-grid 4 may beomitted; and that a ring or disc of size readily calculated by thoseskilledv in the art may be used in place of cylinder 16.

I claim asmy invention: v A

1. An electron gun comprising Va control electrode and a screen-gridelectrode, said electrodes comprising coaxi-al cylinders of insidediameter D having diaphragms .normal to their common axis and withapertures lying on said axis, an electron source on said axis on theside of said control electrode aperture remote from said screen-gridelectrode, a rim of said screen-grid remote from said f control gridlying in a first plane oblique to said axis,

the major length of said screen-grid Ibeing not over .55 D, a firstanode cylinder of diameter D and .having a rim adjacent to saidscreen-grid lying in a seco-nd plane parallel to said first plane andseparated therefrom by a gap substantially 0.20 D Wide, and an electr-onlens at the other end of said first anode cylinder suchthat'substantially all focussing effect occurs in it.

CIL

2. The arrangement specified in claim 1 in which the distance separatingsaid diaphragms is about 0.045 D andthe diameter of said aperturestherein is about .06 D.

3. An electron gun comprising a control electrode and a screen-gridelectrode, said electrodes comprising coaxialcylinders of insidediameter D having diaphragms normal to their common axis and withapertures lying on said axis, an electron source on Isaid axis on theside of said control electrode aperture remote from said screen-gridelectrode, a rim of said screen-grid `remote from said con- -trolelectrode lying in a first plane oblique to said axis, the major lengthof said screen-grid being not over .55 D, a first anode cylinder ofdiameter D and having a rim adjacent to said screen-grid lying in asecon-d plane parallel 4to said first plane and separated therefrom by agap substantially `0.20 D t-o 0.24 D wide, and an electron lens ofsubstantial focussing po'wer at `the other end of said first anodecylinder.

4. An electron gun compri-sing a control electrode and a screen-gridelectrode, said electrodes comprising coaxial cylinders of insidediameter D having diaphragms normal to their common axis `and `withapertures lying -onsaid yaxis, an electron source on said axis on theside of said control electrode aperture remote from said screengridelectrode, a rim of said screen-grid remote from said control electrodelying in a first plane, the length of said screen-grid being not over.44 D, a first anode cylinder of diameter D and having a rim adjacent tos-aid screengrid lying in .a second plane pa-rallel to said first planeand separated therefrom -by a gap of about .20 D wide and an electronlens of substantial focusing power at the other end of said first Ianodecylinder.

5. An electron gun comprising a cathode, a control grid, a screen-grid,a first anode portion, a focusing electrode .and a second anode, -in the`order named, said screen-grid and said first anode comprising lcoaxialcylinders of inside diameter D having diaphragms normal to their commonaxis and with apertures lying on said axis, said screen-grid and saidfirst anode portion forming a first electron lens of given focal lengthon the side of said first lens facing ysaid cathode, said cathode, saidcontrol grid land said screen-grid cooperating to form an electron beamobject for s-aid first lens within the region of said screen-grid, said`screen-grid being of a length of less than .44 D, -a rim of saidscreen-grid remote from said control electrode lying in a first plane.and a rim of said first anode portion adjacent said screen-grid lyingin a second plane and parallel -to said first plane and separatedtherefrom by a gap of about .20 D, the length of said screen-grid Ibeingsuch that the mid plane between said screen-grid and said first anodeportion is at a predetermined distance from said cathode and theprincipal focus of said first lens is beyond said cathode.

References Cited in the file of this patent UNITED STATES PATENTS2,484,721- Moss Oct. 11, 1949 2,496,127 Kelar Jan. 31, 1950 2,515,305Kelar July 18, 1950 2,638,559 Giacchett May l2, 1953 2,680,204 SwedlundJune 31, 1954 OTHER REFERENCES .Fundamentals of Electrical Design (A. D.Moore), published by McGraw-Hill, 1927 (chapters V to IX).

